1. Field of the Invention
The present invention relates to a variable optical attenuator, in particular, which has a variable waveguide for absorbing light to enable attenuation.
In general, the optical signal attenuator for optical communication is an optical component having a set of input and output terminals, by which incident light undergoes a certain magnitude of optical loss to radiate via the output terminal with attenuated optical power.
In optical communication, a system structure varies the level of received optical power which is determined according to the number and performance of optical components such as coupling of optical dividers used in a transmission line, the difference of transmission loss of an optical fiber due to transmission length and the number of optical fiber connecting portions.
If the level of optical input is excessive, an optical attenuator is used to adjust the optical input level. The optical attenuator has other representative uses such as evaluation, adjustment, correction and the like in respect to communication devices or optical measuring devices.
The optical attenuator may be generally divided into a fixed optical attenuator obtaining a fixed amount of attenuation and a variable optical attenuator capable of changing the amount of attenuation. It is necessary for any type of optical attenuator that the amount of attenuation may be not changed by a large amount according to wavelength in a usable range of wavelength.
2. Description of the Related Art
Typical variable optical attenuators of the prior art are divided into a waveguide-type attenuator mainly using a thermo-optic effect of a silicon- or polymer-based material, a large-sized mechanical connector-type attenuator and an MEMS optical-attenuator using an MEMS actuator.
The variable optical attenuators each will be explained as follows.
In general, the waveguide-type variable attenuator has a optical attenuation principle, in which an optical signal is attenuated by forming flat waveguides made of silicon or polymer and changing the temperature distribution of the waveguides by using electrodes while adjusting light absorbing rates of the waveguides. The waveguide-type variable optical attenuator is adequate to small-sized articles, however, they have drawbacks in performance such as large amount of polarization-dependent loss and wavelength dependency.
In the meantime, the mechanical connector-type attenuator achieves optical attenuation according to one of methods, in which an optical fiber is directly deformed to produce transmission loss due to macro bending or the connection distance between transmitting and receiving optical fibers is varied to produce insertion loss. The mechanical connector-type attenuator has a wide range of available wavelength due to no wavelength dependency. However, the above attenuator has disadvantages such as large article size and high cost.
In order to overcome the above disadvantages, the variable optical attenuator using the MEMS actuator has been actively developed. Currently developed MEMS variable optical attenuators include a shutter-type attenuator, a tilting micromirror-type attenuator, a Mechanical Anti-Reflection Switch (MARS) attenuator and the like.
First, the MARS variable optical attenuator functions to adjust the amount of attenuation by positioning a membrane of a mechanical anti-reflection switch based upon the Fabry-Pero principle in an arbitrary displacement rather than On or Off position. The MARS variable optical attenuator has a disadvantage that the amount of attenuation is varied according to wavelength.
The shutter-type MEMS variable optical attenuator has a shutter 103 or 203 placed between a transmitting fiber 101 or 201 and receiving fiber 102 or 202 as shown in FIGS. 1 and 2. The connecting area between the two optical fibers is adjusted according to the displacement of the shutter to control insertion loss. The shutter-type MEMS variable optical attenuator has disadvantages as follows: The first conventional example as shown in FIG. 1 confronts a problem of retroreflection due to an optical signal returning reflected from the shutter; and the second conventional example as shown in FIG. 2 confronts scattering and refraction due to scattered reflection. FIGS. 3 and 4 respectively show effects at the shutters of the variable optical attenuators of the prior art.
Although the shutters of the prior art operate as shown in FIGS. 3 and 4, scattering may take place due to the coarseness and process irregularity of the shutter face. This may cause problems such as retroreflection and scattered reflection in which light propagates across into a transmitting unit or other component by an undesired quantity so that the amount of reflection due to the shutter should be minimized.
The last tilting micromirror-type attenuator connects between the transmitting and receiving optical fibers using reflection of a mirror and controls insertion loss with each displacement of the mirror. The tilting micromirror-type attenuator requires manufacture of a tilting micromirror and a structure of vertically arranging the micromirror on a substrate thereby resulting in a difficult packaging process for assembling the optical fibers by vertically aligning the same in respect to the substrate.
Accordingly the present invention has been made to solve the above problems of the prior art and it is therefore an object of the present invention to provide a variable optical attenuator adopting an MEMS drive mode which prevents efficiency degradation due to scattering of a conventional shutter while simplifies a manufacturing structure in order to improve the performance of a conventional optical attenuator.
According to an aspect of the invention it is provided a variable optical attenuator of the invention comprising: a transmitting fiber for transmitting light; a receiving fiber concentrically arranged with the transmitting fiber for absorbing light; a shutter between the transmitting fiber and the receiving fiber for absorbing light to extinguish absorbed light in areas having no relation to the transmitting or receiving fiber after total-reflection propagation of light inside thereof; an actuator for driving the shutter; and a substrate for supporting the above components.